Posted Sunday, March 4, 2012 @ 10:04 PM
Hiya Bob,
I have replaced the acrylic canopy bubbles on both of my RF4s. Neither was completely successful.
Both frames were a little distorted, so I tried to straighten them. One had previously been broken and brazed, and the other was not quite square and true.
They are made from hard steel with a thin wall -- I would guess 4130, but it is possible to bend each part cold if done with care.
The problem is that, with such a complex shape, I was never able to get the frame shape exactly right. I could get one side to fit perfectly, and then maybe another, but when I tried to get one of the arches right, the opposite end would become skewed.
I just bent the Australian one across my knees, pushing and pulling where needed.
My inspector/buddy/glider repair expert Tony H did the English one, and made a much better job of it, although it still was not quite perfect.
In both cases I just had to accept that the eventual shape I obtained (after a couple of days of wrestling with them) was as good as I was going to get.
I screwed a ready-trimmed, second-hand canopy bubble on to my Australian frame and it worked out fairly well, but the English one did not turn out quite so well.
http://sbeaver.com/cgi-bin/fournier/cutecast.pl?session=jKcRr5wDbtUSAaTqZxaoaLqou6&forum=11&thread=391
The British canopy bubble (from Weiss, with Eugenio's mould) is great, and vision could not be bettered, but the frame no longer fits properly. I have to pull the pin about an inch rearwards before it will drop into its socket, and the bubble edges no longer quite meet the airframe properly all around. What is particularly frustrating is that before finally fitting the bubble, and during the trimming of the bubble and drilling of the fixing screw holes, the canopy frame fitted very well.
Unfortunately, somehow during the final fitting of the bubble the thing got distorted in some way.
One day, when I have seen, closely inspected and photographed Eugenio's canopy, I shall probably remove my British bubble and re-fit it using Eugenio's Sikaflex method.
I could not originally do this because my British inspectors (both of them) wanted the canopy to be attached by its original, certified method -- multiple screws. Now one of my inspectors will accept that its primary means of attachment can be with Sikaflex, so long as some screws are used as a back-up.
In your case Bob, I think I would carefully bend the canopy frame to straighten it as much as possible, although you will probably have to get somebody to replace that right side rail with a straight piece of 4130 (easily bought from Aircraft Spruce) to get the thing right. I think the whole frame is just brazed together, so replacing one rail should not be too difficult.
Then I would clamp the frame into place in the cockpit.
Then I would glue the canopy into place with Sikaflex and leave it to set.
Fit screws at the four corners and perhaps in a couple of places along both side rails as back-ups.
Remove the clamps.
Do all this in the summer and it should be fairly straightforward, but do be warned that old acrylic is quite brittle and much less tolerant of being worked than new stuff.
One heartening thing is that new bubbles are now available from Todd's canopies if you do crack the thing.
Yours, Bob
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